Data link layer protocol for transport of ATM cells over a wireless link

ABSTRACT

The present invention is a reliable data link layer protocol to transport ATM cells over a wireless point-to-point link. The protocol ensures that the cells are transported reliably by use of a sliding window transport mechanism with selective repeat automatic repeat request (ARQ) and forward error correction (FEC). The protocol minimizes ATM header overhead by means of header compression and provides per-cell FEC whose size can be changed adaptively. The protocol also provides parity cells for recovery from errors that cannot be corrected using the per-cell FEC field. The number of these cells as well as the size of a window or frame can also be adaptively changed. In addition, the window can be terminated to request an immediate acknowledgment.

RELATED APPLICATIONS

The present patent application is related to U.S. patent applicationSer. Nos. 08/490,980 and 08/541,984, entitled SIGNALING AND CONTROLARCHITECTURE FOR AN AD-HOC ATM LAN, and METHOD AND APPARATUS FORRESTORATION OF AN ATM NETWORK, those applications having at least onecommon inventor and common assignee and being incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to packet-based telecommunicationsnetworks, and more particularly to a data protocol for ensuring reliabletransmission of data over wireless links in an ATM LAN.

BACKGROUND OF THE INVENTION

ATM (Asynchronous Transfer Mode) technology is maturing rapidly fortelecommunications as well as computer networking applications. Theprospect of an "all ATM" scenario from wide-area network (WAN) tolocal-area network (LAN) is becoming increasingly promising. Discussionson "ATM to the desktop" have begun to appear in various technicalcircles, particularly those interested in multimedia applications.Wireless communications, on the other hand, has gained global acceptanceand popularity in the cellular voice market. Emerging wireless servicessuch as PCS (Personal Communications Service) are threatening to replacetraditional wired telephone and low-rate data access systems. WirelessLAN products (e.g., WaveLan in the Mb/s range) have already found theirway in the commercial marketplace. Extending ATM from the LAN/WANinfrastructure towards the wireless user is a formidable task.

The ATM protocol is intended for transmission on a reliable physicallayer such as optical fibers. Wireless links, on the other hand, arenotorious for their unreliability and poor bit error rates. Thus,overcoming this incompatibility at the onset is a major challenge.

Another important aspect of wireless networking is the user mobility.The whole concept of ATM VP/VC (virtual path/virtual circuit) has beencentered on fixed point end users. Although other research efforts havediscussed the use of ATM in wireless networks, they have been limited tocentralized architectures 10 with wireless access links as shown inFIG. 1. The centerpiece in FIG. 1 is the ATM switch 12 providingcentralized cell routing and mobility management in the entire system.Even though some distributed functions may be assigned to the accesspoints 14, the system intelligence for mobility management resides atthe ATM switch 12. The access points are "hardwired" to the centralswitch while the wireless links serve as extension cords to the users.

It is recognized that wired networks are here to stay, and ATM has thepotential to become ubiquitous. In such a case, there will be standardATM interfaces on workstations, computer servers, and other peripheralsattached to a LAN. Therefore, it is advantageous for a wireless LAN tosupport ATM cell transport directly into the terminals so as to minimizeprotocol conversion. As exemplified in FIG. 2, the wireless LAN 16carries "Wireless ATM" (WATM) whereas the WAN 18 carries (standard) ATM,with a WATM/ATM converter 19 (or gateway) in between. An ideal goal forseamless networking would strive for the elimination of the WATM/ATMconverter 19. This is unrealistic because wireless link layer protocolsneed to be designed differently in order to cope with the poortransmission channel characteristics. Furthermore, this idealistic goalis unnecessary because WATM can be designed essentially the same as ATMexcept for some header byte redefinition and thus keeping the WATM/ATMgateway very simple. A reliable data protocol that preserves the overallATM data structure and minimizes changes in the header would contributesignificantly to the practical realization of Wireless ATM.

SUMMARY OF THE INVENTION

The present invention is a reliable data link layer protocol totransport ATM cells over a wireless point-to-point link. The protocolensures that the cells are transported reliably through the use of asliding window transport mechanism having selective repeat automaticrepeat request (ARQ) and forward error correction (FEC). The protocolminimizes ATM header overhead by means of header compression andprovides per-cell FEC whose size can be changed adaptively. The protocolalso provides parity cells for recovery from errors that cannot becorrected using the per-cell FEC field. The number of these cells aswell as the size of a window or frame can also be adaptively changed. Inaddition, the window can be terminated to request an immediateacknowledgment message (ACK) and to satisfy Quality of Service (QoS)requirements.

One preferred embodiment of the invention includes a data format for usein transmitting ATM cells in a communications network, wherein thecommunications network includes wireless mobile users, and the dataformat comprises: a variable length message frame including one or moreindividual messages, the message frame including, at least onebeginning-of-frame (BOF) message indicative of the beginning of themessage frame; at least one information message including userinformation pertaining to a connection in said network; and at least oneparity message including parity information in regard to the messageframe, and wherein the BOF frame includes one or more fields includingvalues indicative of a number of the individual messages in the messageframe. The message frame may also include an acknowledgment (ACK)message, the acknowledgment message being selectively insertable withinthe message frame to thereby acknowledge receipt of previouslytransmitted information messages and an end-of-frame (EOF) messageselectively insertable within said message frame, the EOF messageindicating premature termination of the message frame thereby overridingthe values provided in the BOF message.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present invention, reference may behad to the following description of exemplary embodiments thereof,considered in conjunction with the accompanying drawings, in which:

FIG. 1 shows a representation of a conventional wireless networkarchitecture;

FIG. 2 shows a representation of an interworking between an ATM WideArea Network (WAN) and a Wireless ATM Network (WATM);

FIG. 3 shows a representation of an Ad-Hoc Wireless LAN according to thepresent invention;

FIG. 4 shows one preferred embodiment of a PBS architecture according tothe present invention;

FIG. 5 shows a one preferred embodiment of a free space optical linkused with the present invention LAN;

FIG. 6 shows an exemplary LAN configuration which illustrates thepresent invention Wireless ATM VP/VC concepts;

FIG. 7 shows a target protocol stack for the network signal flowillustrated in FIG. 2;

FIG. 8 shows an exemplary LAN configuration for illustration of thepresent invention Homing Algorithm routing scheme;

FIG. 9 shows a portion of the network administered tables for use in thepresent invention PBS network;

FIG. 10 shows a connection establishment procedure for the presentinvention wireless ATM LAN;

FIG. 11 shows an exemplary LAN configuration and assignment for theconnection establishment of FIG. 12;

FIG. 12 shows an exemplary routing configuration through the presentinvention LAN;

FIG. 13 shows a connection release procedure for the wireless ATM LAN;

FIG. 14 illustrates a mobile to mobile connection without PBSinvolvement;

FIG. 15 illustrates connection establishment between mobiles on the samePBS; and

FIG. 16 shows a software architecture for PBS and network managementstation.

FIG. 17A shows an exemplary network tree and an associated routingtable;

FIG. 17B shows the network tree of FIG. 17A an the presence of a linkfailure;

FIG. 18A-18F show an exemplary network having different nodes in thenetwork as the root;

FIG. 19 shows an exemplary representation of a failure messageencapsulated into an ATM cell;

FIG. 20A shows an exemplary hardware implementation for determining if areceived message is a failure message;

FIG. 20B shows an exemplary flow diagram for updating routing tables;

FIG. 21 shows exemplary representations of message generated inaccordance with the present invention;

FIG. 22 shows an exemplary representation of an ACK message;

FIG. 23 shows an exemplary representation of a BOF message;

FIG. 24 shows an exemplary representation of an EOF message;

FIG. 25 shows an exemplary representation of information message; and

FIG. 26 shows an exemplary representation of a parity message.

DETAILED DESCRIPTION

The present invention is a reliable data link layer protocol totransport ATM cells over a wireless point-to-point link and is describedwith respect to a LAN consisting of a network of nodes called PortableBases Stations (PBSs) which provide microcell coverage. Referring toFIG. 3, there is shown exemplary representation of the LAN 20 whichincludes a plurality of interconnected PBSs 22. Although theinterconnection between the PBSs can be either wired or wireless, theemphasis here is on wireless implementations, for example, radio orfree-space optics.

Portable Base Station Architecture

An advantage of the Portable Base Station architecture is that the PBSscan employ an ad-hoc networking layout. That is, the PBSs can bedistributed in an arbitrary topology to form a backbone network that canbe reconfigured with relative ease. In FIG. 3 the PBS to PBS backbonelinks 24 are high speed, for example GB/s, for supporting high systemcapacity.

The user to PBS access links 26, on the other hand are primarily formobile access (e.g., 2-20 Mb/s) and therefore are wireless. The mobiles28 which may be comprised of laptops, notebooks, etc. utilize multipleaccess protocols that permit mobiles to share the user to PBS links, aswill be discussed. Here it is also pointed out that mobiles 28 cancommunicate with one another directly as peer-to peer if they are neareach other. Otherwise, they communicate by using the PBS backbone LAN.

As discussed, typical mobile endpoints are assumed to be laptops ornotebook computers. Services supported include conventional dataapplications (e.g., over TCP/IP or SPX over ATM) as well as multimedia(video, voice and data) applications directly over an ATM AdaptationLayer (AAL) and ATM with a Quality of Service (QoS) specification. ForTCP/IP networks, there exist approaches to mobility management at thenetwork layer, e.g., Mobile IP. In the present invention network, theapproach is similar, except mobility management and handoffs are handledat the ATM layer. Thus, the ATM layer becomes the networking layer andmobility management is extended to applications that are directlycarried by AAL and ATM.

Referring to FIG. 4, there is shown one preferred embodiment for a PBSarchitecture 22 according to the present invention. As shown, the PBS 22is comprised of two main components, for example, a PBS VLSI chip 30which contains all switching and interface functions and a PBS processorunit 32. The switching and interface functions of the PBS chip areincluded in a single chip so as to minimize power and space. The PBSprocessor unit 32 is basically a controller that can be implemented withsingle-chip designs as well.

As can be seen, the PBS chip 30 includes a plurality of high speedinterfaces 34 for communicating with other PBSs. The high speedinterfaces 34 transmit, for example, in the Gb/s range and as mentionedpreviously, may be hardwired, but are preferably coupled wirelesslyusing radio and/or optics. The high speed interfaces 34 are coupled toATM switching fabric 36 which is responsible for the physical routingwithin the chip 30. Also coupled to the switching fabric is an optionalATM 38 interface which has connections to and from an ATM network. TheATM switching fabric 36 is coupled to the PBS processor unit 32 by meansof a local port interface 40. The local port interface 40 is in turncoupled to a processor interface 42 which couples to the PBS processor44. Signals to and from the mobile laptops are transmitted and receivedat an antenna 46 or other receive/transmit means where the signalstravel at the Mb/s range. A wireless interface 48 couples the antenna 46to the PBS processor 44. The processor interface 42 may also have adirect connection to the optional ATM interface 38 for control purposes.As will be understood, the PBS will also include memory, for exampleFIFO memory, for selectively storing ATM cells. The memory may store upto a predetermined number of cells and/or may store cells for only up toa given time unit.

Thus, the PBS network is made up of small, high speed ATM switches. Itis an intention to employ off-the-shelf switch fabrics 36, with theaddition of custom hardware and/or software in the PBS backbone network.This requires adhering to ATM standards as much as possible in the PBSbackbone network.

Referring to FIG. 5, there is shown one preferred embodiment of a freespace optical link 50 used with the present invention LAN. The systememploys 1 Gb/s free-space optical links for PBS to PBS communicationsand a 2 Mb/s radio operating at 900 MHz for user to PBS communications.A passive optical lens assembly 52 is used to launch and receive theoptical beam so that the active components can be integrated into thePBS 22 itself. The passive optical unit 52 may be used in conjunctionwith a laser transmitter 54 and optical receiver 56.

An important aspect of the wireless networking scheme utilizing PBSs isthat of user mobility. In addressing this issue, the followingassumptions are made with regard to boundary conditions: (i) slowmobility, for example, walking speed, (ii) distributed control, and(iii) permitted movement through blind spots. A goal of the presentinvention PBS networking scheme is to keep the PBSs simple and low cost.To that end the traditional ATM VP/VC (virtual path/virtual channel) hasbeen modified so as to eliminate the need for any VP/VC translation inthe high speed (Gb/s) portion of intermediate PBS switches. As aconsequence, a "VPI" (virtual path identifier) in the present wirelessLAN scheme corresponds to a particular destination PBS, rather than to avirtual path of base stations and links., where multiple VPIs can beassigned to each PBS.

Referring to FIG. 6, there is shown an exemplary LAN 60 comprised ofPBSs labeled 1-9 and used to illustrate the wireless ATM VP/VC conceptsof the present invention. Mobiles labeled X, Y and Z are shownassociated with PBSs 2, 9 and 1, respectively. The concept of multipleVPI assignments to single PBSs is illustrated in FIG. 6, where VPI 9aand 9b are each assigned to PBS 9. In other words, all cells with VPI 9aand VPI 9b are routed to the destination PBS 9. Associated with each VPI(Destination ID) is a unique route through the network from each source,thus forming a tree with the destination as the root. To distinguishcells from different connections, each PBS controls the assignment ofVCIs associated with VPIs that terminate at that PBS. For example, inFIG. 6, (VPI 9a; VCI 1) and (VPI 9b;VCI 2) denote X to Y mobileconnections, and (VPI 9b;VCI 3) and (VPI 9b;VCI 3) denote Z to Yconnections. The VCI is established by making an inquiry to thedestination PBS using a signaling channel during ATM connectionestablishment.

In order to support mobility within the network, sequence numbers forATM cells will be utilized in certain parts of the network. Note thatsequence numbers are available in the payload of an ATM cell for AAL(ATM Adaptation Layer) 1 through 3/4, but not for AAL 5. This means thatpart of the header field must be utilized for carrying sequence numbers.As the Generic Flow Control (GFC) field is not needed in the PBSbackbone network, the GFC field is chosen to be used for carryingsequence numbers. This leaves a total of 3 bytes to carry theDestination ID and the VCI. These 3 bytes can be divided in any way tocarry the destination ID and the VCI. At the expense of restricting thenumber of PBSs in a backbone network to 256, the Destination ID can beset to 1 byte. This has the advantage of adhering to the ATM UNIstandard. However, as would be understood by one skilled in the art,better use of the 3 bytes may be possible with a "non-standard" divisionbetween Destination ID and VCI.

With the Destination ID set to 1 byte and VCI set to 2 bytes, an ATMnetwork is created for the PBS backbone network where the Destination IDcorresponds to the VPI. As far as the "microstructure" of the PBSoperation is concerned, the operation in the PBS backbone network and anATM network is the same, i.e., an output port is determined based on theinput port and Destination ID, wherein in the above case, no change ismade in the VPI/VCI field. If the Destination ID field is larger than 1byte, although operation is still the same, off-the-shelf ATM switchfabrics may not be used without changes to their VPI/VCI operation.

Another advantage of setting the Destination ID field to 1 byte is thata mobile can communicate with a node outside of the wireless LAN througha gateway PBS without the gateway needing to change anything in theVPI/VCI field of an ATM cell. The ATM signaling protocols allow a nodeconnected to an ATM network to set the VPI/VCI fields for connections toand from the ATM network. With this simplification, functionality of agateway PBS is highly reduced and potentially any PBS can be a gateway.

Referring to FIG. 7, a target protocol stack for the network is shownbelow the signal flow illustration discussed in FIG. 2. As can be seen,the target protocol stack is as follows, Wireless Mobile: CustomWireless, WATM, AAL, IPX, SPX, Application; PBS-Mobile: Custom Wireless,WATM-ATM; PBS-PBS: High Speed Physical Layer, ATM; Wired User: HighSpeed Physical Layer, ATM, AAL, IPX, SPX, Application. In the protocolstack, "Custom Wireless" has a custom physical layer and a custommultiple access control. The custom physical layer has encapsulation ofone or more ATM cells. This encapsulation has a FEC/CRC (forward errorcorrection/cyclic redundancy checking), and may have additional cellsfor cell-level FEC.

Homing Algorithm Routing Scheme

In a mobile environment, handoffs between base stations isstraightforward when the traffic is circuit switched with matchedsource/sink data rates. In contrast, when the traffic is packet-switchedwith source data rates much greater than sink data rates (e.g. many moresources simultaneously transmitting packets to a common destination),there are many more challenges. For example, if ATM cells were alwaysrouted to a mobile's current position, then many cells for a givenmobile may end up queued at other base stations (where the mobilepreviously was positioned). It would then be necessary to retrieve thosecells from those multiple distributed queues and deliver them to amobile in a proper FIFO sequence.

The present invention uses a routing scheme or Homing Algorithm forrouting ATM cells in the wireless mobile network which preserves theproper FIFO sequence, and which allows users to move about the networkand continue communications perhaps even during the middle of an activesession or conversation. Referring to FIG. 8, there is shown anexemplary network configuration in which boxes labeled 1-8 representPBSs 22 (ATM packet switches), and circles A and B denote (two) mobileusers 28 that are communicating with each other. A' and B' denote thelocations of A and B at a later time. A and B have wireless connectionsto the network (specifically at PBSs 2 and 7, respectively) and the highspeed links between PBSs can be either wired or wireless. The term"Local PBS" is used when referring to a PBS associated with a mobile'scurrent position.

In order to maintain reliable, in-sequence ATM transmissions as usersmove during the course of a connection, a "Source Home Station" and"Destination Home Station" are utilized. These Stations refer toparticular PBSs, associated with a connection, that play a pivotal rolein maintaining cell sequence. ATM cells from user A that are destined touser B are first routed from A to the Home PBS for A. The cells are thenrouted along a predetermined virtual path from the Source Home PBS tothe Destination Home PBS, where they are buffered and then deliveredin-sequence to B's Local PBS.

Referring to FIG. 8, a virtual path from PBS 2 to 7 (passing through 4and 6) transports ATM cells from A to B. That is, PBS 2 is the Local PBSfor A and also the Source Home PBS for the A to B connection. Likewise,PBS 7 is the Local PBS for B and also the Destination Home for the A toB connection. When A moves to location A' (with a wireless connection toits new Local PBS 3), the ATM cells are first routed along apredetermined path from PBS 3 back to the Home PBS 2, and then along thevirtual path from 2 to 7. If B has also moved, for example, to B', thenB's Home PBS 7 will forward the ATM cells to B's Local PBS 8, which isits current position, again using a predetermined path.

The advantages of the present Homing Algorithm Routing Scheme includesimple control and preservation of the FIFO cell sequence within a VCand between VCs with common endpoints. The implementation allows thepreservation of the FIFO cell sequence without a centralized controllerand without resequencing at the destination. In the described example,cells obviously preserve their FIFO sequence as they traverse thevirtual path from 2 to 7. Thus, the routing scheme need only maintain"local cell sequence" as cells flow to/from the Home PBS as the handoffoccurs from one base station to the next.

There is some inefficiency associated with always routing cells to/fromHome Stations. Accordingly, to improve network efficiency, the locationsof the Home PBSs are "slowly" updated as users move through the network.For example, after A moves to A', PBS 3 can be redefined to be the newSource Home with, for example, a path through PBS 5 to reach theDestination Home PBS 7.

The following addresses procedures for establishing the updating ofSource Home Local PBSs and Destination Home and Local PBSs. When amobile moves from a first Local PBS S₁ to a second Local PBS S₂, themobile sends a signaling request (through S₂) to its Source Home S_(H)--using the signaling VCI (5). Provided there is bandwidth available tokeep the connection established from the new location S₂, S_(H) respondsthrough S₂ to the mobile with a new VPI and VCI. In addition, S_(H) alsonotifies the mobile of the sequence number of the last ATM cell itreceived from that connection. This is needed to deal with the problemthat ATM cells transmitted while the mobile was at S₁ may not havereached S_(H) yet. Note that this requires S_(H) to record the sequencenumbers of each cell for the active VCs "destined to it" (and thereforeis done on the low-speed side of the PBS), If the last sequence numberreceived at S_(H) is not equal to the last sequence number transmittedby the mobile, then the mobile retransmits the appropriate cells.Meanwhile, S_(H) deletes the old VPI/VCI entry and any remaining cellstransmitted while the mobile was at S₁ will be dropped when they reachS_(H).

If it is desired for this connection to update the Source Home fromS_(H) to the Local PBS S₂, a similar procedure is used, except that thesignaling request is sent to the destination Home D_(H), which thenresponds (if bandwidth is available) with the VPI, VCI, and thelast-received sequence number.

As an alternative to the above-described updating procedure, Sourceforwarding and Source-Home updates may be accomplished in anothermanner. When a mobile moves from Local PBS S₁ to Local PBS S₂, themobile sends a signaling request (through S₂ and S₁) to its Source HomeS_(H). Provided there is bandwidth available to keep the connectionestablished from the new location S₂, S₂ responds to the mobile with anew VPI and VCI. Cells transmitted from S₂ using the new VPI and VCI aredelayed (in a small FIFO) at D_(H) until all cells arrive from S_(H). Aspecial "Tail" Signal sent from S₁ to S_(H) indicates the old path isclear, and also indicates that the old VPI/VCI entry should be deleted.

If it is desired to update the Source Home (for this connection) fromS_(H) to the Local PBS S₂, a similar procedure is used, except that thesignaling request is sent from S_(H) to the Destination Home D_(H),which then responds (if bandwidth is available) with the new VPI andVCI. Cells transmitted directly from the new Source Home S₂ will bedelayed (in a small FIFO) at D_(H) until all cells have arrived fromS_(H) (the previous Source Home) which is indicated by the arrival ofthe "Tail" signal.

When a mobile moves from Local PBS D₁ to Local PBS D₂, (DestinationForwarding) the mobile sends a signaling request (through D₂) to itsDestination Home D_(H). Since D₂ has knowledge of the network topologyand it also controls the assignment of VCIs for connections destined toitself, it includes in the message to D_(H) the new VPI and VCI. Inaddition, the mobile informs D_(H) of the sequence number of the lastreceived cell. If the last sequence number transmitted from D_(H)doesn't match the last sequence number received by the mobile, thenD_(H) retransmits the appropriate ATM cells to D₂ --using the new VPIand VCI. Note that this requires the PBS to have a low-rate "ForwardingQueue" that stores the last "N" ATM cells it forwarded for eachconnection. In addition, there is need for VPI/VCI translation forforwarding cells.

If it is desired for this connection to update the Destination Home fromD_(H) to the Local PBS D₂, a similar procedure is used, except that thesignaling request is sent to the Source Home S_(H) which then (ifbandwidth is available to allow the update) transmits future cellsdirectly to D₂. The mobile then uses a single FIFO buffer to delay (ifnecessary) the cells transmitted to the new Destination Home PBS (D₂)until all cells have arrived from D_(H).

Note that Home PBSs are associated with connections, and not necessarilywith mobiles. For instance the Source Home in FIG. 8 for the connectionfrom A' to B may be different than the Source Home for a connection fromA' to C. Typically, all connections originating from a mobile will havethe same Source Home and all connections destined to a mobile will havethe same Destination Home. However, because Home stations can be updatedas mobiles move about the network, various connections may temporarilyhave different Homes.

Finally, for "forwarding cells", note that VPI/VCI translation isrequired only at the Source Home S_(H) (if the source is not located atS_(H)) and the Destination Home (if the destination is not located atD_(H)).

Control and Management

In this section, certain control and management functions areconsidered, i.e., network management, mobility management, andconnection control. Network management algorithms are designed toconfigure data tables that are used by connection control algorithms.These data tables change due to network upgrades and faults/restorals.Two aspects of mobility management are considered: (i) registrations, tohandle mobile sign-ons, and idle handoffs, i.e., handoffs that occurwhen a mobile unit is powered on, but not in a connection; and (ii)mobile location procedures during connection setup. Finally, connectioncontrol procedures are described to set up and release connectionson-demand. This includes managing available resources, such as linkbandwidth and VCIs. Since the present inventions LAN is targeted for usewith multimedia end-units, to ensure guaranteed jitter bounds for voiceand video (isochronous) traffic, schemes are adopted whereQuality-of-Service (QoS) guarantees are provided during connectionadmission. For applications that do not require such QoS guarantees,simplified connection control schemes can be applied by omitting the QoSchecking phase.

Some of the signaling messages, needed to support the connection controland mobility management procedures, are sent on out-of-band signalingchannels, i.e., the standard signaling VCI (VCI 5) on all VPIs. Othermessages are exchanged on inband signaling channels (using the same VCIas the user information, with the Payload Type (PT) field distinguishingsignaling data from user traffic), or on dedicated VCIs.

In order to administer the per PBS VPI allocations described in FIG. 6VPI routing tables are needed to be set up in the PBSs as shown in FIG.9. The VPIs allocated to each PBS are shown in italics within boxesmarked with the PBS identifiers (numbers I-IV). Within the presentsystem, it will be understood that VPI 0 is reserved formobile-to-mobile communication. Each PBS stores the VPI routing TableM₁, which is a mapping of an incoming VPI on any port to an outgoingPort. In FIG. 9, a part of this table is shown for PBS IV. In setting upTable M₁, a unique path from any source to any destination PBS isensured for a given VPI. The algorithms for configuring and maintainingthese tables can be executed in a centralized or distributed manner.

In addition, VPI 100 is assigned as a broadcast VPI used by all the PBSsfor mobile location prior to connection establishment. Using shortestpath algorithms, such as Dijkstra's or Bellman-Ford, a tree is set upfrom each PBS to all other PBSs. Different VCIs are used to distinguishthe trees originating from different PBSs all of which use VPI 100.Table M₂, as shown in FIG. 9, is needed to map/translate VCIs only forVPI 100 and other local VPIs. The table entries corresponding to VPI 100only map input port and VCI to an output port without changing the VCIs.For data received on local VPIs, both VPI and VCI may be translated. Tomaintain the ad-hoc nature of this network, the number of networkmanagement administered tables that are required in each PBS has beenminimized while designing control procedures.

Next, procedures are described for two aspects of mobility management:registrations (for power-up, power-down, and idle handoffs) and mobilelocation. Three options are considered for mobile registration andlocation. In Option I, each mobile registers on power-up and power-down,as well as when it moves to a new PBS. Each PBS broadcasts itsidentifying beacon which allows the mobile to determine that it hasentered the domain of a new PBS. When a PBS receives a registrationmessage, it broadcasts the mobile's presence to all the other PBSs inthe LAN. Thus, if a connection is requested to this mobile, the LocalPBS of the calling mobile knows the location of the called mobileimmediately. A connection can be set up from itself to the Local PBS ofthe called mobile.

In Option II, each mobile registers on power-up, power-down, and onchanging PBSs, but the PBS, upon receiving a registration message, doesnot broadcast the presence of the mobile to the other PBSs. Thus, when aconnection is requested to a mobile, the Local PBS of the calling mobiledoes not know the current location of the called mobile or even if it ispowered-on. A page is required to find the Local PBS of the calledmobile.

In Option III, a mobile does not register at all. In this case, when aconnection request is made, the Local PBS of the calling mobile mustinitiate a page to all the PBSs. Each PBS, in turn, must initiate a pageto determine if the called mobile is listening to its beacon.

In a preferred embodiment of the invention, Option II is selected forthe PBS LAN. This selection is based on the trade-off between theexcessive connection setup time needed in Option III vs. the excessivenumber of messages needed to track the mobile in Option I. Thus, ourmobile location algorithm or method consists of using broadcast pages tolocate the Local PBS of the called mobile.

Idle handoffs occur when a powered mobile, that is not part of aconnection, moves from one PBS to the next. Registration messages aresent by a mobile whenever it identifies a new PBS. The beacontransmitted by the PBS includes as a parameter one of its assigned VPIs,thus enabling the mobile to recognize the presence of a new PBS.Assuming that the strength of the signal from the mobile to the old PBSmay be weak, instead of requiring the mobile to de-register with the oldPBS, the new PBS sends a De-registration message to the old PBS.

The procedure for mobile location prior to connection establishment isnow described. As can be understood, the process of requiring a mobileto register upon powering-up allows the PBS-based network to set upincoming connections to the mobile. Upon receiving an incoming messagefrom a mobile, the Local PBS simply makes a record of the mobile. Itdoes not register the mobile's presence in any location database, as isdone in current cellular networks, nor does it broadcast the presence ofthe mobile to the other PBSs. Thus, when a connection is requested to amobile, the Local PBS of the calling mobile does not know the currentlocation of the called mobile or even if it is powered-on. A broadcastpage is generated by the Local PBS of the calling mobile to find theLocal PBS of the called mobile. If the called mobile is registered onone of the PBSs in the network, this PBS responds to the broadcast pageand connection establishment between the two Local PBSs of the callingand called users proceeds. If the mobile is not registered at any of thePBSs in the network, the Local PBS of the calling mobile that generatedthe broadcast page times-out and rejects the call request.

We do not employ any location servers or registers in order to keep thenetwork simple. As we are primarily focused on a LAN, when a mobilegenerates a request for connections to another mobile, the Local PBS ofthe calling mobile merely performs a broadcast to determine the PBS atwhich the called mobile is registered. The called mobile's Local PBSresponds, allowing for the connection setup between the two PBSs toproceed. For mobile-to-fixed-endpoint (server or user) connections, aPBS may store the identity of the PBS to which the fixed endpoint isconnected or may determine the location each time by thebroadcast-search process.

Connection control is now described, wherein it is shown how connectionsare set up in this LAN while supporting the new Wireless ATM VP/VCconcept. All the scenarios discussed are for two-party connections.Extensions of these procedures for multi-party connections andthird-party connection control may also be derived.

The steps involved in setting up on-demand connections in any ATMnetwork consist of:

1. Finding a route between the endpoints of the connection,

2. Checking the availability of bandwidth and other QoS measures, ifany,

3. Selecting VCIs at each link on the end-to-end connection, and

4. Setting up VP/VC and port-translation tables.

In the present invention LAN, step 1 is not required for each on-demandconnection. By using destination-based VPI addressing, the routes arepredetermined by the mapping tables M₁ at each PBS as previouslydescribed. In step 2, the availability of bandwidth and other QoSmeasures are checked at each transit PBS on the route between the LocalPBSs of the calling and called mobiles. The selection of VCIs in step 3,and the setting up of translations in step 4 (mapping table M₂), need tobe performed only at the two Local PBSs. Before performing steps 2,3,and 4 to establish a connection, a procedure is needed to locate thecalled mobile since this ATM LAN is primarily used to interconnectmobiles.

The procedure for locating the called mobile, selecting VCIs, checkingthe availability of the requested QoS, and setting up translations isexplained by example. Referring to FIG. 10, a bi-directional connectionsetup initiated by mobile A to mobile B is illustrated. As can be seen,mobile A generates a Setup-connection message with parametersidentifying the two mobiles and QoS measures, if any. The Local PBS ofmobile A, which is PBS I in FIG. 10, generates a broadcast message, onVPI 100 and its assigned VCI, in order to locate the called mobile'sLocal PBS. In this message, besides the called mobile's address, itassigns one of its VPIs (VPI 1a) with a VCI (VCI 4) for the backwardconnection, i.e. from the called mobile's Local PBS to itself, as shownin FIG. 11. Assuming the called mobile B is located on PBS IV, this PBSoffers the connection to mobile B. This step is needed to alert thecalled mobile before actually setting up the connection. If this isaccepted, PBS IV responds to PBS I with a Mobile-located message inwhich it assigns a VPI (VPI 4a) and VCI (VCI 10) for the forwardconnection from PBS I to PBS IV. FIG. 11 shows that the selected VPIsfor the two directions of the connection may follow different routes,i.e. pass through different intermediate PBSs. The destination PBS foreach connection is assumed to manage the VCIs incoming on its VPIs. Ifthe called mobile (mobile B) rejects the connection offer, aConnection-rejected message is sent from PBS IV to PBS I, andsubsequently to the calling mobile (mobile A).

Referring to FIG. 11 in connection with FIG. 10, it can be seen thatnext, both PBS I and PBS IV send the Check-QoS message in oppositedirections to check for the availability of the QoS measures requestedon the routes followed by VPI 4a and VPI 1a, respectively. The Check-QoSmessages are sent in a hop-by-hop manner through all the PBSs in thepath between the two Local PBSs for the assigned VPIs. Unlike B-ISDNsignaling standards for ATM switches, inband signaling is used in thisLAN to carry this Check-QoS message. Upon receiving this message, eachtransit PBS, such as PBS III, determines if the requested QoS measuresare available. For example, if average bandwidth is one of the QoSmeasures specified, PBS III would check for the availability of therequested bandwidth on its outgoing port for the VPI being traced. Ifthe requested QoS measures are available, the transit PBS passes on thein-band signaling message Check-QoS to the next PBS on the route for theVPI on which the message arrived, after reserving the required QoSmeasures for the given connection. The two PBSs (PBS I and PBS IV)exchange QoS-available messages upon the successful reception of theCheck-QoS messages. Although the QoS parameters supported in thisnetwork have not been explicitly stated, depending on theimplementation, these could include peak and average bandwidth, delay,jitter, etc.

In-band signaling implies using the VPI and VCI of the assignedconnection with the payload type (PT) field indicating a signaling cell.For example, the Check-QoS message from PBS IV to PBS I is sent on VPI1a, VCI 4 with the PT field set to indicate signaling. Parameters ofthis message are defined such that the whole message does not requiremore than one ATM cell. This eliminates the need for a signaling AAL andspeeds up the processing of this message at each transit PBS. Using theVPI/VCI of the assigned connection, with the payload-type in the ATMcell header set to indicate signaling, the message is passed through theset of transit switches on the assigned VPI. Each PBS checks foravailability of the specified QoS measures, and passes the message tothe next PBS on the VP tree. By using in-band signaling, we reduceprotocol layer processing at each transit PBS. It also implies thatadditional data tables are not required at the PBSs for routingsignaling messages. For example, if the Check-QoS message is sentout-of-band, then the message from one transit node to the next needs tobe sent on the signaling VCI of a VPI assigned to the receiving transitPBS. This PBS then needs to consult a data table to determine a VPI forthe next transit PBS that is on the route of the VPI for the connectionbeing traced. Thus, with a hybrid out-of-band and in-band signalingscheme, connection control and mobility management procedures can besupported with minimal data tables.

As in FIG. 11, it is likely that in our LAN the set of transit switchesin the two directions may be different because VPI trees may not bepre-configured with symmetric routes. Another example of this case isshown in FIG. 12. This implies that QoS checking must be done separatelyin the two directions. In the present invention LAN, we have theadvantage that the two Local PBSs are determined before connectionestablishment. For connection requests to mobiles, this determination ismade by both Local PBSs during the broadcast-location phase. Forconnection requests to fixed endpoints, the calling party's Local PBSmay determine the far-end Local PBS simply from the called party'saddress. It then communicates its own identity and the need for aconnection establishment to the far-end Local PBS. This feature allowsthe QoS checking process to proceed in both directions simultaneously.

At this point, still referring to FIGS. 10 and 11, the two Local PBSssend the Set-endpoint messages communicating the VPI/VCI pair for theforward and backward connections to the two end-mobiles. Each Local PBSreuses the VPI/VCI incoming to itself from the far-end Local PBS on theair interface link. But, for the opposite direction, it assigns aVPI/VCI corresponding to one of its own VPIs. For example, PBS I reusesVPI 1a, VCI 4 for its downward link to the mobile, but picks a VPI/VCI(VPI 1b, VCI 1) terminating on itself for the upward link.

If a PBS in transit finds that it cannot allocate the requested QoS, itsends a QoS-unavailable message on the signaling channel (VCI 5) of theVPI being traced. This message directly reaches the end PBS without anyprocessing in the intermediate nodes. In this case, this PBS (being oneof the two Local PBS 6) sends a QoS-unavailable message to the otherLocal PBS. The connection is rejected to the requesting mobile by itsLocal PBS and the VCIs and any reserved QoS measures are released.

In order to speed-up the end-to-end connection setup process, eachtransit PBS can send the Check-QoS forward to the next PBS while itperforms its own processing. To support this, separate positive (andnegative) messages need to be generated by each transit PBS to the endLocal PBS to confirm QoS availability. The increased messaging needs tobe traded-off against the gain in end-to-end connection setup delay forspecific network designs.

The procedure used to release connections is shown in FIG. 13. It can beinitiated by either of the mobiles A or B in the connection with aRelease-connection message. The Local PBS of the release-initiatingmobile sends a Free-connection-resources message. It also sends aDrop-endpoint message back to the mobile. The far-end PBS (PBS IV) sendsa Drop-endpoint message to its mobile and generates aFree-connection-resources message to the first transit PBS in the routealong the backward connection. The transit PBSs (PBS II and PBS III)release the resources that they had reserved on the two routes (VPI 1aand VPI 4a). These messages are routed inband on the VP in VCIs beingreleased.

An alternative method to set up fast connections with QoS guarantees isdescribed in the following paragraphs. Each PBS stores the routes fromany source PBS to itself for each of its designated VPIs. Link and noderesources are pre-divided among all the PBSs. Thus, a PBS whileassigning VCIs for incoming on-demand connections, checks resourceavailability on all the links and nodes in the route from the source PBSto itself for the given VPI selected. Connection setup time is lowerusing this approach when compared to the approach presented earlier,since the hop-by-hop Check-QoS procedure is no longer needed. Forexample, in FIG. 11, resource availability on all the links and nodesconstituting the route from PBS I to PBS IV is checked at PBS IV and onthe route from PBS IV to PBS I at PBS I. Drawbacks of this approachinclude a reduction in network scalability and a reduction instatistical multiplexing gains. Hybrid schemes combining the twoapproaches can also be considered for large networks.

To support the approach described earlier with respect to FIG. 11, asimple data table M₃ is needed at each PBS to track its own resources,such as bandwidth on its ports. For each on-demand connection that isestablished across a PBS, the PBS determines if it has sufficientresources to meet the QoS requirements of the connection. To support thealternative approach presented here, we need two mapping tables in eachPBS X, these tables are:

M₃ : Destination VPI+Source PBS ID→a route of PBSs; for all VPIsassigned to the PBS X and where the route is specified using a sequenceof PBS IDs and port numbers on the PBSs; and

M₄ : link/node→allocated resource; for all links and nodes in thenetwork. Maintaining these additional tables, however, is difficult inan ad-hoc network. Thus, it may necessitate the need for a systemadministrator while upgrading the network, i.e., when a PBS is added orremoved.

As described previously, the present invention PBS network allowsmobiles outside the range of the PBS network to communicate directlywith each other. When a mobile powers-on, it checks to see if it canpick up a transmitting beacon of some other mobile or a PBS. If no suchmobile or PBS exists, it starts transmitting its own beacon, and thusbecomes a master 70 as shown in FIG. 14. When a second mobile 72 listensto this beacon, it can request connections to other mobiles as shown inFIG. 14. The master mobile 70 allocates VCIs for the two channels in abi-directional connection using Set-endpoint messages. It is assumedthat VPI 0 is used for mobile-to-mobile connections, as describedearlier. Such connections are handled in small disjointmobile-controlled groups outside the scope of the PBS network.

A distinction needs to be drawn between the case discussed above and onein which a PBS is present with both mobiles on the connection beinglocated on the same PBS. In the latter case, the procedure discussedusing FIG. 10 applies. In FIG. 15, there is shown a mobile 75 (mobile A)originating a call to another mobile 77 (mobile B) that is also in thedomain of the same PBS 79. The message exchange consists ofSetup-connection, Offer-connection, Connection-accepted, andSet-endpoint, for connection establishment, and Release-connection, andDrop-endpoint, for connection release. The cross-PBS mobile location andQoS checking procedures are not required in this scenario. The userinformation connection may be routed through the PBS or directly betweenthe two mobiles, as shown in FIG. 15, depending on whether or not, thetwo mobiles are within listening distance of each other. In FIG. 15, thebi-directional user connection is within the control of PBS I, and henceuses one of its VPIs, VPI 1a.

The layout of the networking software needed for control and managementin the PBSs and network management stations is shown in FIG. 16. Eachblock shown within the PBS processor 80 and network management station82 in FIG. 16 represents a software functional entity. Each entity canbe implemented as a single process or a collection of processes. The PBSAgent 84 in each PBS tracks free and allocated VCIs on all the VPIsassigned to the PBS. In addition, it also manages resources, such aslink bandwidth, for the PBS. The Mobility Manager 86 processesregistrations, generates/responds to broadcast location messages, andmanages handoffs. The Connection Manager 88 is needed only in thealternative connection establishment approach described above. It checksresource availability on the route taken from a source PBS to a givendestination on any of its VPIs during on-demand connection establishmentfor connections that require QoS guarantees.

The software entities on each network management station 82 include aConfiguration Manager 90 and a Network Resource Distributor 92. Theconfiguration manager 90 sets up the mapping tables, M₁, and part of M₂,(for VPI 100), as defined with respect to FIG. 9, and handles theaddition and deletion of PBSs, allowing the LAN to grow in an ad-hocmanner. The configuration manager 90 may also be implemented in adistributed manner at each PBS, depending on the processing and memorycapabilities available in the PBSs. In this case, a network may beconstructed with only PBS elements, i.e., without any network managementstations. The network resource distributor 92 pre-assigns resources ofeach node and link in the network to all PBSs, i.e., sets-up tables M₃and M₄ described in the alternative connection establishment procedure.The exact number of network management stations needed is dependent onthe traffic and configuration of the PBS LAN. Distributed algorithms forthese functions may be implemented across multiple network managementstations that are connected to one or more PBSs. This modularizedsoftware architecture allows a network to be constructed using only thePBS agent, mobility manager and configuration manager modules. Such anetwork would only support the connection establishment proceduredescribed in FIG. 13. To offer connection services with improved setupdelays, the network resource distributor and connection manager modulesare needed. These can be added at an additional cost.

Network Reliability

When considering the reliability of the present wireless LAN, issues canbe divided into two categories, the reliability of the PBS backbonenetwork and the reliability of the air link connecting the PBS to amobile station. In the PBS backbone network, the sources of failures arecomponent (transmitter, receiver, or whole PBS) failures, occasionallink failures (due to obstruction of the radio or optical signal, orfiber or cable cuts in the cases of fiber and cable transmission), andsoftware failures that may affect a group of PBSs. With transmissionsover the air link, errors result from the multi-path and noise problems,which should he combated with a good design at lower layers, inparticular, the physical layer and the data link layer.

The division of data transmission into one or more Mb/s unreliablewireless access links and the Gb/s reliable backbone links for thiswireless LAN facilitates the division of the reliability problem intothe above two subproblems. Restoration and reliable transmission methodsdeveloped for the ATM networks, however, do not automatically carry overto this network.

This is because the PBS backbone network has ATM cells in transport, andis similar to an ATM network, except, as opposed to a regular ATMnetwork, destination routing is used. ATM restoration algorithms thatemploy rerouting or splitting of routes simply by changing addresses incell headers do not automatically apply, and different restorationtechniques or different implementations of existing restorationtechniques of ATM networks need to be developed.

Additionally, the air interface carries packets made up of one or moreATM cells. The basic design philosophy in ATM, an extremely reliableunderlying network, is violated over the air. Therefore, the argumentthat an ATM network should support end-to-end error control does notcarry over to this wireless LAN, although it has a transport mechanismsimilar to ATM. Error control is best accomplished at the air-PBSnetwork boundary for two reasons. First, in the presence of awireless-to-wireless connection with the PBS backbone network inbetween, the end-to-end error control becomes extremely difficult due tothe possibility of loss of messages and acknowledgments in two separateunreliable links (the same argument applies to flow control). Repetitionor redundant transmission of packets over a reliable air link due to anunreliable counterpart at the other end does not make sense since thebandwidth over the air is an expensive resource. Secondly, once thecells are inside the PBS backbone network, they are transmitted overhighly reliable links, and link-by-link error control then becomesmeaningless (again, the same argument applies to flow control).

Also, to keep the PBSs as simple and inexpensive as possible, complexrestoration algorithms cannot be employed in the PBS backbone network.This wireless network represents a different paradigm than the cellularnetworks in existence today, which can support complicated base stationequipment. In such networks, typically, mobiles have smaller processingpower. Protocols have been designed to exploit this asymmetry and movethe complexity of operation into the base station. The instant goal,however, is to design as simple PBSs as possible for economical reasonsas well as for reasons of scalability to larger networks. Therefore,transmission protocols such as AirMail, where the base station does mostof the processing, are not appropriate. In fact, since the asymmetry isreversed with respect to cellular networks, if an asymmetric protocol isto be employed, it should be the mobile that does most of theprocessing.

A. Network Restoration

Restoration issues in modem LAN, MAN, or WAN networks are consideredhighly important, and are usually addressed from the onset. For example,FDDI has dual rings which are configurable into a single ring in thecase of link failure. Node failures are addressed by bypass options atthe MAC layer. The IEEE MAN standard 802.6 addresses restorationsimilarly, and has provisions for reconfiguration into slot generationstations for any network node in the case of link failure. ATMrestoration issues are being standardized by means of OA&M signalhierarchies known as flows, and alarm and notification signals known asAIS (Alarm Indication Signal) and FERF (Far-End Receiver Failure) atdifferent levels of the flow hierarchy.

The principal routing technique in this network between PBSs, however,is destination routing which presents a distinct variation from theabove. Accordingly, for every destination node, a tree (or possibly afamily of trees) is generated that connects all PBSs. The cells carrydestination addresses at their headers, and routing decisions at eachPBS are made based on the header address. The decision involves a simplelook-up from a local routing table that maps each header to an outputport. During network booting, the look-up tables are generated by arouting algorithm. Various centralized or distributed algorithms can beemployed for this purpose, such as the Bellman-Ford algorithm, or theminimum spanning tree algorithm. In considering the problem ofrestoration, a primary interest is in the updates to the routing tablesat each PBS in the case of a failure.

Referring to FIG. 17, a tree 110 and associated routing table are shown.First, it should be illustrated that in the case of a link failure,updating the routing tables at an affected transmitter node is notsufficient. This point is illustrated in FIG. 17A for the routing tree110 of FIG. 17. As can be seen, when the affected PBS reroutes trafficas shown, a cycle 112 is generated in the tree, since the traffic isdeflected to a PBS that transmits incoming cells back to the affectedPBS in accordance with the original tree designation which is, ofcourse, undesirable.

The optimum remedy to the problem is in calculating the optimum tree asif the failed link were disconnected, and updating the routing tables ateach PBS. Various centralized or distributed algorithms exist for thispurpose. Some of the applicable centralized and distributed alternativesare described.

A first issue is the detection of failure. In the case of full duplexlinks, a failure is detected by both ends of the link failure. However,common failures involve transmitter and receiver failures in the case ofoptical links, and it is therefore safe to conclude that the failuresshould be detected by the receiving PBS.

A second issue is the propagation of this information to an agent thatwill make the rerouting decision. In the case of a centralized reroutingalgorithm, this agent is the central decision maker. In the case of adistributed algorithm, the information (in the form of a Failure ID)should be broadcast to all the PBSs. As the failure information isreceived by each node, rerouting tables are updated. For LANs, prestoredrerouting tables indexed by a failure ID at each PBS are a suitableoption. In this case, after detection, the failure ID is broadcast, andafter a time-out period that allows for the message to reach all basestations, all base stations use the new routing table indexed by thefailure ID, stored at network boot time. For MAN and WAN applications,the information is transmitted to the central controller, whichcalculates the new routing tables and informs each PBS of the updates inits routing tables. As would be understood, these restoration algorithmswill be implemented using the software architecture describedpreviously.

The following is a more detailed description of a network restorationscheme in the present ad-hoc ATM LAN. The present LAN associates afailure ID with the failure of each node or link therein. The specificfailure ID is provided at the time the system is generated, and is knownby every node in the system. This failure ID identifies the failednetwork element, i.e., a network link or a network node, by means of asingle bit and indicates the transfer of its state from good to bad. Bymeans of this bit there is also an indication of a transition from badto good which is needed for recovery.

As can be understood, each node in the system has a view of the networkas a tree where that node is at the root of the tree. FIG. 18illustrates an exemplary network 120a having nodes A-E, wherein avariety of minimum spanning trees are shown. Network configurations120(b)14 (f) show minimum spanning trees having node A-E, respectively,as the root. These trees have the property that every node on it is atthe shortest distance to the node, and all nodes in the network are onthe tree. Such trees are known as minimum spanning trees. Generation ofthese trees from a given network topology is well-known in the art. Moregenerally, in the present ad-hoc ATM LAN, any tree which spans all ofthe (PBS) nodes can be used. In the present network, each tree is givenan ID number or "tree ID". Messages traversing this tree are identifiedby the tree ID, wherein the tree ID is mapped to an output port by arouting table at each intermediate node.

Referring to FIG. 19, there is shown an exemplary representation of afailure message 130 as the message is encapsulated into an ATM cell. Thefailure message 130 includes a VPI/VCI field 132 within the ATM cellheader 134, wherein a predetermined VPI/VCI value is reserved forfailure reporting to all nodes, for example, the value "1000". Thefailure message includes an ATM cell payload 136 which includes a fieldfor the failure ID 138 and the ID of the tree (tree ID 140) the messageneeds to traverse. Based on the tree ID 140, the message is replicatedon one or more output ports, or if it is a terminating node, the messageis not propagated any further.

When the message 130 is received, an intermediate network node or PBSlooks at the VPI/VCI field and characterizes the message as a failuremessage based on the specific value of the VPI/VCI field 132, forexample 1000. The node then examines the payload 136, determines thefailure ID 138 and based on the tree ID 140 determines the port to whichthe message should be propagated, or else, when the node is aterminating node, if the message should be propagated any further. Anadditional field included in the payload is a time stamp field 142. Thetime stamp 142 indicates the time the failure was initially detected bythe node that determines the failure. A switch will then be made to newrouting tables by all nodes in the network at a fixed time after thisstamped time. The fixed time is chosen such that during this time, themessage can propagate to all nodes in the network. The selection of thisfixed time is important in that it should be long enough to allow themessage to traverse the network, even under fully loaded conditions.However, the duration should also be short enough for fast recovery fromfailures. This time interval is set at the network generation time.

In order to minimize the time it takes for a failure identificationmessage to travel through multiple software layers, a hardwarearchitecture is included in the nodes that identifies a failure message130 and caries out the above-described steps. Referring to FIG. 20A, anexemplary representation is shown for a hardware device 150 to determineif an incoming ATM cell is a failure message 130. The device includes anATM cell buffer 152 and a second buffer 154 for storing the VPI/VCIfailure value. A failure indicator 156 checks if the VPI/VCI field 132of the incoming ATM cell is the same as the predetermined failureVPI/VCI, e.g., 1000. For this purpose, each bit in the buffer iscompared with the prestored value in the second buffer 154. If the bitis the same as the prestored bit, the modulo-2 summation generates azero. The failure indicator 156 checks for all zeros to determine if thecell belongs to a failure message. It will be understood that othercomparison schemes may also be utilized for comparing the VPI/VCI fieldwith a predetermined failure value.

Referring to FIG. 20B, an exemplary algorithm is shown for updating ofthe routing tables within the network. Arrival of each ATM cellinitiates the algorithm using the box "Incoming cell" 160. Many suchoperations can be performed in parallel. In addition, when the responseto the failure decision box 162 is "yes", it will be understood that twoparallel operations are performed as shown.

As discussed above, after the incoming cell is received, a PBS comparesthe VPI/VCI field with a predetermined value to determine whether or notthis is a failure message 162. If the VPI does not indicate a failurevalue, an output port is determined 164 based on the VPINCI. The cell isthen transferred to that output port 166. If it is a failure messagethat has been received, an output port is then determined 168 based onthe tree ID 140 found in the failure message 130. After the output portis determined based on the tree ID, the cell is transferred to thatoutput port 166.

After the failure message has been recognized, a second paralleloperation is accomplished to update the routing tables in the network.Initially, the specific failure ID 138 is stored within the PBS node orsomewhere else in the network 170. Based on the failure ID, the networkidentifies the failed element. Based on the failed element there arethree alternatives for a node to perform 172. A first is that the nodemakes a calculation of the new routing table using a standard algorithm,such as Dijkstra or Bellman-Ford algorithms which are known in the art.Second the routing table is calculated at the network generation timeand prestored. That is, for each failure ID, a complete new routingtable exists in storage and the network pulls up that routing table.Third, the calculation is made or stored by an agent for a group ofnodes and the information is propagated to the groups of nodes concernedby a signaling channel.

Once the new routing tables are calculated/recalled or received at step172, the network waits a predetermined time period 174 for the failuremessage to traverse the network. Once this time has elapsed, the routingtables within the network are then updated 176.

B. Link Error Control

Error control in the present wireless LAN is achieved by means ofphysical layer and link layer protocols. As was stated above, the desireto make the PBS as simple as possible suggests an asymmetry in theprotocol with the burden of complexity placed on the mobile, and not onthe PBS. This places a slight additional power burden on the mobile forthe processing and transmission of control messages. However, for amobile terminal capable of processing multimedia messages with theassociated encoding and decoding operations, this additional burden isnot significant.

Error control can be achieved at the physical layer and at the data linklayer. Although some arguments exist for leaving error control to higherlayers, a decision is made to terminate the data link layer at theaccess-backbone boundary. This provides better utilization of the twoindependent wireless links, and prevents unnecessary retransmissionsover a clean wireless link at a source for example, because of errors onits noisy destination counterpart.

An important consideration for this LAN is the degree of error controlcoding required. A significant number of cases would involveline-of-sight transmission without multipath fading and therefore,error-free transmissions. When that is the case, there is no need forany error control coding in the physical layer or in the data linklayer. However, as is well known, a wireless link with multipathreception is prone to high error rates due to Rayleigh fading. In thatcase, heavy error control would be needed. Correspondingly, an adaptiveerror control scheme is opted for, where channel measurements are madebased on message acknowledgments, and error control overhead is adjustedaccordingly.

Typically, the physical layer is forward error controlled in hardware.For the physical layer, two options can be considered: no error control,which would be the case for line-of-sight applications, and asingle-rate forward error control in hardware. To flexibly accommodateboth scenarios, triggers from one state to the other are implementedbased on link layer measurements using acknowledgments. Similarly, forthe data link layer, the forward error correction overhead is changedbased on acknowledgments of correctly received data.

The present invention discloses a reliable data link layer protocol totransport ATM cells over a wireless point-to-point link. As has beendiscussed, it is understood that a wireless link can have a large numberof errors, whereas ATM is designed for very reliable media, such asoptical fiber. The present protocol ensures that the cells aretransported reliably by a sliding window transport mechanism withselective repeat automatic repeat request (ARQ) and forward errorcorrection (FEC). The protocol minimizes ATM header overhead by means ofheader compression and provides per-cell FEC whose size can be changedadaptively. The protocol also provides parity cells for recovery fromerrors that cannot be corrected using the per-cell FEC field. The numberof these cells as well as the size of a window or frame can also beadaptively changed. In addition, the window can be terminated to requestan immediate acknowledgment message (ACK) and to satisfy Quality ofService (QoS) requirements.

A requirement of the sliding window-type protocol with selective repeatmechanism is that a large buffer may be needed in the mobile (or PBS) inorder for cells to be delivered in sequence. With today's operatingsystems that deliver buffer space on request, this limitation should beeasily overcome. The following describes basic elements of the protocol,it will be understood, however, that additional messages can be addedwithout altering the basic operation, and without altering the maincontribution.

Referring to FIG. 21, there are shown four exemplary representations210, 220, 230, 240 of messages in accordance with the present inventiondata protocol. The protocol transfers messages in a window or framecomprised of one or more ATM cells, wherein the frame may consist ofthree distinct messages: control, information or parity. Frame 210 isrepresentative of a "standard" frame as viewed within the presentinvention protocol. Frame 210 includes a Beginning of Frame (BOF)message 212 which is considered a control message, 1 through Ninformation messages (I) 214 and 1 through M parity messages (P) 216. Ascan be seen, the control, information and parity messages (or cells) arecontained within a single window or frame 218. The size of each messagewithin the frame may be deduced from the bits at the beginning of amessage cell, as will be explained. In this way, a recipient candetermine what to do to process the next message. As will be understood,the size of the window is adaptively varied depending upon thereliability of transmissions in the data link at any one time. Theprotocol is also full duplex.

A second exemplary frame 220 includes a BOF message 212 followed by afirst section 222 of 1-N information messages 214. Another type ofcontrol message, an acknowledge message (ACK) 224 follows the firstsection 222 of information messages. The ACK message 224 is followed bya second section 226 of 1-N information messages 214 followed by 1-Mparity messages 216. The ACK message, as will be explained, is used toacknowledge the status of received cells. In the case of frame 220, theACK message 224 may be used for acknowledgments on the reverse linkduring transmission of a standard frame. Frame 230, on the other hand,is representative of an isolated ACK message 224, which may also be sentindependently.

A fourth exemplary frame 240 includes and End of Frame message (EOF)242. The EOF message 242 may be inserted within a frame in order tosignify premature termination thereof. As can be seen in frame 240, aninformation section 244 of the frame includes less than N informationmessages 214 or cells, wherein the frame is terminated by the EOFmessage. The EOF message is followed by 1 through M' parity messages. Aswill be explained, the number of information and parity cells N and M,respectively, is set in the BOF message 212, whereas in the fourth frame240, the number of parity cells 216 transmitted after an EOF message isset in the EOF message.

The specific format for each of the above messages will now be describedin greater detail. Referring to FIG. 22, there is shown one preferredrepresentation of a block acknowledgment (ACK) control message 224. TheACK message is transmitted from the receiver of a transmission to thetransmitter of that transmission in order to acknowledge the receipt ofmessages. As has been described previously, there are three types ofcontrol messages, ACK, BOF and EOF. Each control message begins with acontrol message identified field 250 (1 bit) that is equal to 0, therebyidentifying the message as a control message. A second field 252 is anACK identifier, also 1 bit, which is set to 0 so as to identify thismessage as an acknowledgment message. As will be understood, the size ofthe message depends on the size of the frame or window which is known byboth the transmitter and the receiver based on prior informationexchange.

Following the identifier field is a frame acknowledgment bit map 254.The bit map in the ACK message includes a 1 for each information cell(ATM cell) that is correctly received, and a 0 for an ATM cell that isnot correctly received within the window. An ATM cell that was in factreceived in error, but whose errors are corrected using the per cellFEC, or by means of the parity cells is considered an ATM cell that iscorrectly received for the purposes of the acknowledgment message. Thesize of the bit map 254 is determined based on the number of informationcells (I) 214 in the frame, and this is known to both the transmitterand the receiver. An FEC field 256 is appended to the ACK messagefollowing the bit map 254. The FEC field 256 includes a forward errorcorrection code for correcting errors in the transmission. Forward errorcorrecting is well known to those skilled in the art, and codes such asReed-Solomon and Erasure codes may be utilized, for example, in thiscircumstance.

Referring to FIG. 23, there is shown an exemplary representation of asecond type of control message, the beginning of frame message (BOF)212. The BOF message 212 is transmitted at the beginning of each frameas was illustrated in FIG. 21. Following the control message identifierfield 250 is the BOF identifier field 260. This field 260 is comprisedof 2 bits, a first being set to a 1 and a second being set to a 0 foridentification purposes of the BOF message. The BOF message 212 includesthree additional fields, an information number message field 262, aparity message number field 264 and a FEC byte number field 266. Theinformation number message field 262 indicates the number of informationcells 214 in the frame. The parity message number field 264 indicatesthe number of parity cells in the frame, and the FEC byte number field266 indicates the number of bytes used in the per-cell FEC. In order tominimize the overhead for these fields, information is transmittedincrementally. For example, two bits (having 2² states) may be used toidentify any one of the following four cases: (i) default value, (ii)same value as previous value, (iii) increment previous value, and (iv)decrement previous value. It will be understood that more bits may beadded to each field in order to accommodate several default values andseveral increment/decrement values. For example, with a three bit field,three default values, two increment and decrement values each, and ano-change message can be accommodated, wherein it is understood thatother variations are also possible. The BOF message 212 also employs aFEC field 256 following the FEC byte number field 266. The FEC field 256is always constant or else, is given in the previous BOF.

Referring to FIG. 24, there is shown an exemplary representation of anend of frame field (EOF) 242. This kind of control message is used toindicate the end of a frame when it is wished to terminate a frameprematurely, that is to override the value provide by BOF. Besides thecontrol message identifies field 250, the EOF includes a 2 bitidentifier field 270, wherein the first and second bits in this fieldare set to 0 and 1, respectively. An updated parity number message field272 follows the identifier field 250. This field indicates the number ofparity cells to be appended after the EOF message 242, since that numbermay have changed because of the termination of the frame. The EOFmessage 242 employs a FEC field 256 whose size is given by the previousBOF.

Referring to FIG. 25, there is shown one preferred exemplary embodimentof an information message 214 in accordance with the present inventiondata protocol. Each information message 214 includes an information cell282 which is prepended by an identifier field 280 containing multiplesubfields. Depending on the identifier field, the information cell 282will be a complete ATM cell, minus the header error correction (HEC)field, or only the payload of an ATM cell. A first subfield is thecontrol message identifier field 250. In this case the control messagefield 250 is a 1 bit field, wherein a 1 identifies the message as anon-control message. A second field is a header compression field 284which is used to compress the header overhead. In its simplest form, theheader compression field 284 is a single bit. For one value of this bit,for example a 1, the VPI/VCI field of this cell is equal to that of theprevious cell and, therefore, the header is not transmitted. For theother bit value, for example 0, the VPI/VCI field is not equal to thatof the previous cell and, therefore, the full ATM cell (minus the HECfield) is transmitted.

More generally, the header compression field 284 will include n bits,wherein a PBS or mobile station include 2^(n-1) registers that holdVPI/VCI information. A first bit in the field will then indicate a setor read command. When a predetermined value of this bit is used to set aregister, a VPINCI field of the ATM cell is stored in the register, andthe full ATM cell (minus the HEC field) is transmitted. When this field284 is used to read a register, the addressed register's VPI/VCI fieldis recalled from memory at the receiver, and therefore only the payloadof the ATM cell is transmitted over the air.

As an example, a "0000" header compression field 284 contents mayindicate to set Register 0 (of an 8 register field 0 through 7) with theVI/VCI value of the existing cell. A "1000" would indicate to readregister 1. Continuing with the same example, a "0001" would indicate toset Register 1 with the VPI/VCI of the existing cell, wherein a "1001"value would indicate to read Register 1. As mentioned, in the aboveexample, 8 registers are included 0 through 7, however, it will beunderstood that any number of registers n may be utilized using similarheader compression concepts. Here, whenever a set register value isused, a full ATM cell (minus the HEC field) is transported. After an ATMcell is transported, and its VPI/VCI is identified either bytransmission or through memory, the appropriate HEC field is added atthe receiver. Another single bit subfield is included in the identifierfield 280, this is the ACK required field 286. A predetermined value ofthis field 286 would be used by the transmitter to indicate that anacknowledgment on this cell is not expected from the receiver. Thiswould be used, for example, with real-time message. As with each of theother messages in the present invention protocol, a per cell FEC field256 is appended to the ATM cell within the information message.

Referring to FIG. 26, there is shown an exemplary representation of aparity message 216. As was illustrated in FIG. 21, parity messages 216are placed in the frame after the last information cell 214 or the EOFcell 242. Parity cells 290 are constructed using the payloads ofinformation cells 214 and therefore their size is equal to 48 byteseach. Parity cells 290 need not be prepended by the beginning of cellfields for parsing purposes, however to be able to insert an ACK messagein between parity cells, an identification field 250 is included toidentify the messages as non-control messages. A single bit controlmessage field 250 is illustrated in FIG. 26 which is prepended to the(48 byte) parity cell. A per-cell FEC field 256 is appended to theparity cell. As would be understood, parity cells are not acknowledged.

It will also be understood that the forward error correction is achievedat three different levels, (bit-level, byte level and packet-level) bymeans of operations of the physical layer and the data link layer.Bit-level FEC is accomplished by physical layer error control andprovides protection against random bit errors.

Byte-level FEC is accomplished by FEC overhead on each air link packet.Air link packets already have CRC overhead in order to determine whetherthey have been received in error. Here, a more powerful code, such as aReed-Solomon code, is utilized and correction of errored packets withoutretransmissions is accomplished, based simply on the data inside thepacket.

Packet-level FEC is accomplished by means of additional packetstransmitted over the air, and is useful when method the byte-level FECabove does not suffice to reconstruct a packet based on the overhead inthe packet. This level of FEC operates in conjunction with the automaticrepeat request (ARQ) mechanism previously described. For real-timeapplications, such as voice or video, FEC only is preferable. For dataapplications, FEC operates in conjunction with retransmissions.

These methods provide increasing protection against burst errors, withmore bursts being combated at each level, but with the tradeoff ofhigher complexity and increased delay. The bit-level is implemented inhardware, therefore the issue of complexity is essentially irrelevant.This is because it is turned on or off in order not to waste bandwidthunnecessarily.

The byte and packet-levels are implemented in software. The byte-levelreplaces the CRC check function, which should be performed even in theabsence of FEC. Therefore, increased complexity due to FEC is tolerableand it is suggested placing this functionality symmetrically at the baseand at the mobile. The packet-level requires the largest complexity andmemory, and it is suggested using this for base-to-mobile transmissions,since decoding is the computationally intensive part of FEC.

From the above, it should be understood that the embodiments described,in regard to the drawings, are merely exemplary and that a personskilled in the art may make variations and modifications to the shownembodiments without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A data format for use in transmitting ATM cellsin a communications network, wherein said communications networkincludes wireless mobile devices, said wireless mobile devices havingmemory for storing and processing said ATM cells using said data format,said data format comprising:a variable length message frame includingone or more individual messages, said message frame including,at leastone beginning-of-frame (BOF) message indicative of the beginning of saidmessage frame; at least one information message, said informationmessage including user information pertaining to a connection in saidnetwork; and at least one parity message selectively inserted in saidmessage frame when error recovery is required, said at least one paritymessage including parity information in regard to said message frame,wherein said BOF message includes one or more fields including valuesindicative of a number of said individual messages in said messageframe.
 2. The format of claim 1, further including an acknowledgment(ACK) message, said acknowledgment message being selectively insertedwithin said message frame when acknowledgment of receipt of previouslytransmitted information messages is required.
 3. The format of claim 2,further including an end-of-frame (EOF) message selectively insertedwithin said message frame when said message frame is to be prematurelyterminated, said EOF message overriding said values provided in said BOFmessage.
 4. The format of claim 1, wherein said message frame includescontrol messages containing message frame processing information andnon-control messages, wherein each of said individual messages includesa control message field at the start thereof, a first predeterminedvalue in said control message field being indicative of a controlmessage and a second predetermined value being indicative of anon-control message.
 5. The format of claim 1, wherein each of saidindividual messages includes an adaplively alterable forward errorcorrection (FEC) field, and wherein said BOF message includes separatefields indicative of a number of a specific type message, said separatefields including:an information message number field indicative of anumber of information messages included in said message frame; a paritymessage number field indicative of a number of parity messages includedin said message frame; and an FEC byte number field indicative of anumber of bytes in said FEC field for each said individual message. 6.The format of claim 5, wherein information in said separate fields ofsaid BOF message is transmitted incrementally based on a specific valuecontained in each of said separate fields, wherein:a first predeterminedvalue indicates a corresponding default value for each of said separatefields respectively; a second predetermined value indicates a same valueas a previously transmitted value corresponding to each of said separatefields; a third predetermined value indicates an increment of a previousvalue corresponding to each of said separate fields; and a fourthpredetermined value indicates a decrement of a previous valuecorresponding to each of said separate fields.
 7. The format of claim 4,wherein said control messages further include a message identifier fieldfollowing said control message field for identifying a specific controlmessage.
 8. The format of claim 2, wherein said ACK message includes aframe acknowledgment bitmap, wherein the size of said bitmap is based ona number of information cells in said message frame, said bitmapincluding a predetermined bit value for each said information messagethat is correctly received.
 9. The format of claim 3, wherein said EOFmessage includes an updated parity message number field indicative of anupdated number of parity messages to be appended to said message frame.10. The format of claim 1, wherein said information message includes aheader presence field, wherein a first predetermined value of saidheader presence field indicates that an ATM cell included within saidinformation message will be transmitted without a header, and a secondpredetermined value of said header presence field indicates that a fullATM cell is transmitted.
 11. The format of claim 1, wherein said networkincludes switches which contain registers having VPI/VCI informationstored therein, wherein said information message includes an n-bitheader compression field, wherein a first predetermined value of a firstbit of said header compression field indicates to set a registeridentified by said header compression field in one of said switches,wherein a VPI/VCI field of an ATM cell in said information message isstored in said register and a full ATM cell is transmitted; andwherein asecond predetermined value of said first bit of said header compressionfield indicates to read a register identified by said field, wherein aVPI/VCI field stored in said register is recalled and an ATM cell insaid information message is transmitted without a header.
 12. The formatof claim 2, wherein said information message includes an acknowledgmentrequired field indicative of whether an acknowledgment message isexpected from a receiver of an ATM cell included in said informationmessage.
 13. The format of claim 1, wherein said parity message includesa 48 byte parity cell.
 14. A data protocol for use in transmitting ATMcells in a communications network, wherein said communications networkincludes portable base station (PBS) switches configurable into anad-hoc backbone network, said PBS switches adapted to communicate withwireless mobile devices, said PBS switches and said wireless mobiledevices having memory for storing and processing said ATM cells usingsaid data protocol, said data protocol comprising:a variably sizedmessage window, said message window including,at least one controlmessage including processing information regarding said message window,wherein variable combinations of control, information and paritymessages are selectively inserted within said message window based onservice requirements, and wherein specific ones of said control messagesare indicative of the size of said message window.
 15. The data protocolof claim 14, wherein said control messages are selected from the groupconsisting of beginning of frame (BOF), end of frame (EOF) andacknowledge (ACK) messages, wherein:a BOF message is indicative of thebeginning of said message window; an ACK message acknowledges receipt ofpreviously transmitted information messages and is selectively insertedwithin said message window if desired; and an EOF message indicatespremature termination of said message frame thereby overriding valuesprovided in said BOF message.
 16. The protocol of claim 15, wherein eachof said messages includes a forward error correction (FEC) field, andwherein said BOF message includes separate fields indicative of a numberof a specific type message, said separate fields including:aninformation message number field indicative of a number of informationmessages included in said message window; a parity message number fieldindicative of a number of parity messages included in said messagewindow; and an FEC byte number field indicative of a number of bytes insaid FEC field for each of said messages.
 17. The protocol of claim 16,wherein information in said separate fields of said BOF message istransmitted incrementally based on a specific value contained in each ofsaid separate fields, wherein:a first predetermined value indicates acorresponding default value for each of said separate fieldsrespectively; a second predetermined value indicates a same value as apreviously transmitted value corresponding to each of said separatefields; a third predetermined value indicates an increment of a previousvalue corresponding to each of said separate fields; and a fourthpredetermined value indicates a decrement of a previous valuecorresponding to each of said separate fields.
 18. The protocol of claim15, wherein said ACK message includes a frame acknowledgment bitmap,wherein the size of said bitmap is based on a number of informationcells in said message frame, said bitmap including a predetermined bitvalue for each said information message that is correctly received. 19.The protocol of claim 14, wherein said information message includes aheader presence field, wherein a first predetermined value of saidheader presence field indicates that an ATM cell included within saidinformation message will be transmitted without a header, and a secondpredetermined value of said header presence field indicates that a fullATM cell is transmitted.
 20. The protocol of claim 14, wherein saidnetwork includes switches which contain registers having VPI/VCIinformation stored therein, wherein said information message includes ann-bit header compression field, wherein a first predetermined value of afirst bit of said header compression field indicates to set a registeridentified by said header compression field in one of said switches,wherein a VPI/VCI field of an ATM cell in said information message isstored in said register and a full ATM cell is transmitted; and whereina second predetermined value of said first bit of said headercompression field indicates to read a register identified by said field,wherein a VPI/VCI field stored in said register is recalled and an ATMcell in said information message is transmitted without a header.
 21. Amethod for transmitting ATM cells in a communications network, whereinsaid communications network includes portable base station (PBS)switches configurable into an ad-hoc backbone network, said PBS switchesadapted to communicate with wireless mobile devices, said methodcomprising the steps of:assembling a variable length message frameincluding one or more individual messages for transmission within saidnetwork; transmitting at least one beginning-of-frame (BOF) messageindicative of the beginning of said message frame; transmitting at leastone information message, said information message including userinformation pertaining to a connection in said network; and selectivelyinserting at least one acknowledgment (ACK) message in said messageframe to thereby acknowledge receipt of previously transmittedinformation messages, and wherein said BOF message includes one or morefields including values indicative of a number of said individualmessages in said message frame.
 22. The method of claim 21, furtherincluding the step of selectively inserting parity messages in saidmessage frame including parity information in regard to said messageframe.
 23. The method of claim 21, further including the step ofselectively inserting an end-of-frame (EOF) message within said messageframe, said EOF message indicating premature termination of said messageframe thereby overriding said values provided in said BOF message. 24.The format of claim 21, wherein each of said individual messagesincludes an adaptively alterable forward error correction (FEC) field,and wherein said BOF message includes separate fields indicative of anumber of a specific type message, said separate fields including:aninformation message number field indicative of a number of informationmessages included in said message frame; a parity message number fieldindicative of a number of parity messages included in said messageframe; and an FEC byte number field indicative of a number of bytes insaid FEC field for each said individual message.
 25. The method of claim24, wherein information in said separate fields of said BOF message istransmitted incrementally based on a specific value contained in each ofsaid separate fields, wherein:a first predetermined value indicates acorresponding default value for each of said separate fieldsrespectively; a second predetermined value indicates a same value as apreviously transmitted value corresponding to each of said separatefields; a third predetermined value indicates an increment of a previousvalue corresponding to each of said separate fields; and a fourthpredetermined value indicates a decrement of a previous valuecorresponding to each of said separate fields.
 26. The format of claim21, wherein said ACK message includes a frame acknowledgment bitmap,wherein the size of said bitmap is based on a number of informationcells in said message frame, said bitmap including a predetermined bitvalue for each said information message that is correctly received. 27.The format of claim 21, wherein said information message includes aheader presence field, wherein a first predetermined value of saidheader presence field indicates that an ATM cell included within saidinformation message will be transmitted without a header, and a secondpredetermined value of said header presence field indicates that a fullATM cell is transmitted.